Major League Soccer’s generational shift lurched forward with another retirement of a league leader on Tuesday, as veteran midfielder Pablo Mastroeni called time on a 16-year career.

Widely known as a physically imposing presence in the middle of the field and considered one of the best holding midfielders of his generation, the 37-year old Mastroeni appeared in 334 regular-season games since his league debut in 1998, fifth all-time among field players.

“I am honored to have spent my entire professional club career playing in MLS and I now look forward to watching this league continue to grow as I begin the next stage of my life," Mastroeni said in a statement. "I have been lucky to cross paths with so many kindred spirits, from teammates to opponents, coaches and staff, and especially fans.

"I am indebted to all the coaches I've had in my life, who influenced me to become a mentor to the next generation of players in this country. Finally, thanks to the Colorado Rapids and their wonderful fans for providing me the best years of my career, and the faith they showed in me to help bring an MLS Cup to Colorado.”

Although best known for his days as a gritty midfielder with Colorado and the US national team, Mastroeni earned some of his first accolades as a defender with the star-studded Miami Fusion. In fact, Mastroeni earned the only MLS Best XI selection of his career as a defender during the Fusion’s memorable final season in 2001, when they won the Supporters’ Shield.

When the team was contracted following the season, Mastroeni was the first overall pick in the 2002 Allocation Draft to the Rapids, who leaned on his midfield play and promptly made five consecutive playoff appearances. After a three-year hiatus from the postseason they returned in 2010, when Mastroeni played every minute of a surprising run that took the team from a wild card berth to an MLS Cup championship.

“Pablo’s contributions to the [Rapids] can’t be overstated, and we all want to thank and honor him for his service to the Rapids,” said Colorado technical director Paul Bravo. “Pablo was a true competitor on the field, and over his time with Colorado, we all saw what a powerful impact he had on his teammates and on the fans. He’s one of the true greats of American soccer and will always be a Colorado Rapids legend.”

Mastroeni was hampered by injuries late in his career, most notably a concussion sustained early in the 2012 season that limited him to just two appearances that year, the fewest of his career. He appeared in just 16 total games for the Rapids and Galaxy in 2013.

Mastroeni retires with the dubious but fitting distinction as the all-time league leader in yellow card cautions (81), just nine ahead of former Fusion and Rapids teammate and current Real Salt Lake captain Kyle Beckerman. Mastroeni is also second all-time in red cards (8) and tied for third all-time in fouls committed (474).

Mastroeni was a regular for the US national team from 2001-09, logging 65 career caps and appearing in both the 2002 and 2006 World Cups. A late addition to the 2002 team after an injury to midfielder Chris Armas, Mastroeni started three of the team’s five games: a group stage win over Portugal, a Round of 16 win over Mexico and a quarterfinal loss to Germany.

Mastroeni returned in 2006 and started the team’s first two group games, but was sent off during the team’s 1-1 draw against Italy for a rash tackle late in the first half on Andrea Pirlo.

“Pablo is a special player who made every team he was on better. His presence, work ethic and enthusiasm for the game made him a successful player on both the club and national team level,” former USMNT and current LA Galaxy head coach Bruce Arena said in a statement.